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Multiple Carb Transporters - rethink your sport nutrition


The ideal fueling strategy during training/racing is the consumption of carbohydrates in a way that can be quickly emptied from the stomach, be rapidly digested and quickly absorbed so it can be used by the working muscles.  Just because you are consuming a gel, bar, solid food or drink with carbohydrates, this doesn't mean that your muscles are receiving those carbohydrates. The takeaway from this is not all carbs are the same.

Exercise shifts blood flow away from the GI (Gastrointestional tract) towards the active muscles and lungs. Digestion is compromised during exercise. This is why it's important that your carb choices during exercise do not require a lot of digestion. The quicker and easier those carbs are emptied from the stomach, the quicker those carbs can be used by the active muscles. Also, the more digestion that is required, the greater risk for GI issues. 

After digestion comes absorption. For the muscles to continue to perform during long distance activity, absorption moves nutrients, water and electrolytes from the small intestines into the cell and then into the blood. Because cell membranes are careful not to let dangerous substances into the body, they make it difficult for nutrients to enter the body. Therefore, nutrients need the help of a transporter (protein) to move across the cell membrane barrier. For carbohydrates to be absorbed, a transporter takes the digested and broken down carbohydrate from the intestional lumen inside the intestine, through the intestinal wall and into the body circulation. 

There are two specific protein transporters (SGLT1 and GLUT5) that allow for carbohydrate absorption. When your sport drink contains glucose, sucrose, galactose, maltodextrin or starch, exogenous carbohydrate oxidation peaks ~60g/hr (~240 calories). This is because the SGLT1 transporter becomes saturated ataround 1 gram of carbohydrate per minute. This is very important because if your sport drink, bar, or gel contains only the carbohydrates listed above, consuming more than 60 grams per hour will not result in more carbohydrate oxidation. The excess carbohydrate will not be absorbed and will accumulate in the intestines. This means the muscles will fatigue despite continuing to consume a large amount of carbohydrates and the risk for GI issues increases dramatically.

Because fructose uses a different transporter (GLUT5), the additional of fructose to a sport drink will allow for higher oxidation rates (up to 90g/hr) so long as you saturate the SGLT1 transporter with 60 grams of glucose or maltodextrin. Because these carbs use different transporters, you can deliver more carbohydrate to the muscle per hour.

If you are simply consuming carbohydrates without the awareness of how much you are consuming, what types of carbs you are consuming and how often you are consuming them, there's a good chance that you are not optimizing absorption - which means a greater risk for bonking, early fatigue, dehydration and GI issues. 

According to research, the ideal combination of carbs include: 
  • maltodextrin : fructose 
  • glucose : fructose 
  • glucose : sucrose : fructose
If you are trying to consume more than 60 grams of carbs per hour (which is around 240 calories), you need to consume a product with multiple transportable carbohydrates so that you don't clog the SGLT1 transporter. 

Because most endurance athletes will benefit from consuming between 60-90g of carbohydrates on the bike per hour, I put together the helpful chart above to help take away the guessing when trying to plan how to fuel based on the duration of your workout/race.

To increase the capacity to absorb carbohydrates, it's critical that your sport drink has the right formulation. Simply eating/drinking whatever you want, whenever you want, does not guarantee that what you are consuming is being digested and absorbed.